1. Field of the Invention
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 2003-030425 filed in JAPAN on Feb. 7, 2003, the entire contents of which are hereby incorporated by reference.
The present invention relates to a projection-type display apparatus for use in a projection television or the like, and a method of driving such a projection-type display apparatus.
2. Description of Related Art
A conventional projection-type display apparatus has, in addition to three light valves corresponding to three primary colors, an extra light valve for enhancing brightness of a projected image. The brightness of the image can be increased by adding up the lights exiting from these four light valves (for example, refer to Patent Document 1). Another conventional projection-type display apparatus has a shutter disposed across an optical path of illuminating light in order to improve contrast (for example, refer to Patent Document 2 and Patent Document 3). Another conventional projection-type display apparatus has three reflective liquid-crystal light valves for displaying images in high resolution and good contrast (for example, refer to Patent Document 4).
On the other hand, for providing low-cost projection-type display apparatuses, it is desirable that they have only one light valve. There are known several techniques for realizing such a projection-type display apparatus excellent in brightness while having only one liquid-crystal light valve (for example, refer to Non-Patent Document 1 and Non-Patent Document 2). Non-Patent Document 3 describes the image-quality problem in displaying moving pictures by use of a hold-type light valve in which image data is held until next image data is input, and a remedy to this problem.
[Patent Document 1]
    Japanese Patent Application Laid-open No. 8-294138 (Pages 5-6, FIG. 1)[Patent Document 2]    Japanese Patent Application Laid-open No. 8-21977 (Pages 3-4, FIG. 2)[Patent Document 3]    Japanese Patent Application Laid-open No. 2001-100699 (Pages 3-4, FIG. 1)[Patent Document 4]    Japanese Patent Application Laid-open No. 2002-62583 (Pages 4-5, FIG. 1)[Non-Patent Document 1]    Serge Bierhuizen, Single Panel Color Sequential Projectors with Polarization Recovery, SID'02 Digest-55.5[Non-Patent Document 2]    D. Scott Dewald, Steven M. Penn, M. Davis, Sequential Color Recapture and Dynamic Filtering: A Method of Scrolling Color, SID'01 Digest, page 1076-1079 (2001)[Non-Patent Document 3]    T. Kurita, SID2001 Digest, 986, “Moving Picture Quality Improvement for Hold-type AM-LCDs”
The light valves for use in the projection-type display apparatus suitable for displaying images in large size are roughly classified into two types. One type uses a self-emission CRT (Cathode Ray Tube), and the other type uses a non-self-emission light valve. The latter is suitable for displaying images in high brightness and high resolution. In the projection-type display apparatus of the type using the non-self-emission light valve, its light valve is irradiated by illuminating light which is emitted from a lamp as a light source and condensed by a light condensing device. The light valve modulates the illuminating light in accordance with image signals, and outputs the modulated light to a projection device such as a projection lens. The projection device throws the modulated light onto a screen or the like.
It is desired that the light source is small in size to gain high utilization efficiency of light. For example, an ultra-high pressure mercury lamp is used for the light source. The non-self-emission light valve may be a transmissive liquid-crystal light valve, a reflective liquid-crystal light valve, or a micro mirror device. It is said that the reflective liquid-crystal light valve can have finer pixels required for high-resolution display with relative ease, since it can be manufactured by a process similar to semiconductor-manufacturing processes. Important characteristics of the projection-type display apparatus includes brightness, contrast and resolution. It is also important the projection-type display apparatus can display moving images clearly when it supports motion-picture signals such as television signals.
Contrast affects impression of image quality of a projected image greatly. In the conventional projection-type display apparatus using the light valve of the non-self-emission type, the contrast ratio between a full screen black pattern and a full screen white pattern (referred to as “full-white to full-black contrast” hereinafter) is a few hundreds to one. However, the contrast ratio between a high-brightness area and a low-brightness area in an actual image (referred to as “image contrast” hereinafter) falls short of the full-white to full-black contrast due to stray light within the display apparatus etc. Accordingly, there has been a problem that a projected image looks undesirably whitish especially in a dark movie scene. Meanwhile, the full-white to full-black contrast of the projection-type display apparatus using a CRT that emits light itself is as large as a few thousands to one. As explained above, the conventional projection-type display apparatus using the light valve of the non-self-emission type is not at a satisfactory level in image contrast.
The projection-type display apparatus with a light shutter disposed across its optical path can adjust an average brightness of a projected image by having the light shutter control intensity of the light illuminating the light valve, whereby the full-white to full-black contrast is improved. However, the image contrast itself cannot be improved by using such a light shutter.
With the projection-type display apparatus which modulates red, green, blue and white lights individually by light valves, and adds the modulated lights together, peak brightness of a projected image can be increased, however, the image contrast itself cannot be improved. Besides, when the white light is added to the red, green and white lights for enhancing brightness, color reproduction range (gamut) is narrowed, and color tone (intensity level) of an input image is not reproduced faithfully in some cases.
The projection-type display apparatus having only one light valve can be manufactured at lower cost. However, it has a problem that the utilization efficiency of light is low and brightness of a projected image is therefore low when projecting a full color image, since the full color image is projected on a field-sequential basis in which the lights of colors which are not selected in each field are discarded. It is possible to improve brightness by using a high-power lamp. However, in this case, a large lamp has to be used and a large-scaled cooling structure has to be provided. In consequence, the display apparatus becomes large in size and manufacturing cost increases contrary. In the case of using a liquid-crystal device as the light valve, there is a problem that leakage light increases, since it is necessary to pass a wide-wavelength-band light through a polarization-separation prism, which makes the image contrast low.
Most of the light valves of the non-self-emission type are hold-type light valves in which image data is held until the next image data is input. However, there is a problem that the projection-type display apparatuses which use such a hold-type light valve is inferior to the display apparatuses which use a CRT in image quality when projecting moving pictures (for example, refer to Non-Patent Document 3). Non-Patent Document 3 discloses, for overcoming this problem, having the lamp blink, or providing periods during each of which the light valve forms a full screen black pattern. However, the former involves the disadvantage that the life span of the lamp is shortened, and the latter involves the disadvantage that brightness is lowered. Accordingly, it was difficult to apply such countermeasures to the projection-type display apparatus using the hold-type light valve.